A chemiresistor is a particle composite whose electrical conductivity changes in the presence of particular chemical vapors known as analytes (typically volatile organic compounds). The magnitude of this observed resistance change is related to the concentration of analyte vapor in the environment. In general, the chemiresistor composite can comprise a plurality of conductive particles in a continuous electrically insulating matrix. Chemiresistor composites traditionally comprise carbon-black particles, which are randomly distributed in an amorphous (non-crosslinked) polymer matrix.
One type of chemiresistor device is known as a field-structured chemiresistor (FSCR). FSCRs contain particles that have a magnetic core and a conductive shell and are fabricated by mixing a particular volume fraction of these particles with a viscous elastomer precursor. This particle suspension is then subjected to a magnetic field and the particles form a structured network due to their dipole-dipole interactions. The elastomer is then cured and the magnetic field is removed, resulting in a permanently structured, electrically conductive composite. Field structuring brings the particles to the electrical conduction (percolation) threshold largely independent of particle volume fraction, whereas traditional chemiresistors require a high volume fraction of particles to conduct. Field-structuring of the particle phase can improve the response and reproducibility of the sensors. It is noteworthy that FSCRs can employ a cross-linked elastomer as the continuous matrix of the composite. FSCRs employing cross-linked elastomers have the useful property of reversible strain. With an elastomeric matrix, a particular analyte concentration does not necessarily irreversibly swell the matrix, and the matrix can shrink when the analyte concentration is reduced. This is often not the case with other types of chemiresistors and is one of the contributing factors to FSCR reversibility. FSCRs can exhibit high sensitivity, excellent sensor-to-sensor reproducibility, negligible irreversibility, and increased baseline stability.
When a chemiresistor is exposed to chemical vapors, the composite swells due to the absorption of these vapors into the polymer matrix. This swelling-induced isotropic strain causes the particle network to pull apart, breaking electrically conducting pathways and diminishing inter-particle contact pressure, thereby increasing contact resistance. This leads to a measurable increase in the composite's overall resistance. The observed resistance change is correlated to the concentration of analyte in the environment through the transduction or response curve. The response of an FSCR can be described as a function of analyte concentration in terms of its relative conductance ratio, G/Go, where Go is the conductance of an FSCR in the absence of analyte and G is the conductance at a particular analyte concentration (at thermodynamic equilibrium). Plotting G/Go as a function of analyte concentration results in a sigmoidal curve as seen in FIG. 1(a). FSCR sensitivity is the slope of the transduction curve at a specific analyte concentration. It is seen from FIG. 1(b) that the sensitivity of an FSCR varies over the range of analyte concentrations; the maximum sensitivity occurs around the inflection point of the curve (mid-point response or G/Go=0.5), whereas at the upper and lower limits of the sensing range the sensitivity is extremely low.
With conventional chemiresistors, obtaining sufficient sensitivity over an extended range of concentration requires having several different chemiresistors, each comprising a film permanently tuned to a different portion of the extended range. A single conventional chemiresistor is not able to provide sensitivity over a wide range of analyte concentrations because range tunability was not a characteristic of conventional chemiresistors.
Therefore, a need remains for a single chemiresistor that can provide sensitivity over a wide range of analyte concentrations.